Silva suffered the setback in Wednesday's 3-1 UEFA Champions League quarter-final first leg defeat to Barcelona. The Brazil centre-back limped off in the 21st minute at the Parc des Princes.
"Thiago Silva is out for Nice even if his injury is not as bad as first feared," Blanc said.
Turning his attention to midfielder Thiago Motta, who has been out for the last 12 days with a similar problem, Blanc added: "Thiago Motta is also out even though he is not far from returning to the squad."
PSG will need to quickly refocus on Saturday's encounter following the disappointment of midweek as they bid to keep pace with Lyon at the top of the Ligue 1 table.
Lyon are two points clear at the summit, although they have played a game more than PSG.
Blanc said: "It is not easy to prepare for this match. We must turn in up in championship mode. In this final stretch, we no longer have room for error."
